A defense expert's testimony Wednesday challenged the prosecution's claim that teenage driver Joseph Beer was impaired by marijuana during a 2012 Southern State Parkway wreck that left four friends dead.

Yale School of Medicine psychiatry professor Mehmet Sofuoglu said in the case of "chronic" marijuana smokers, the level of the active ingredient THC in their blood isn't a good measure of whether they're impaired.

Sofuoglu, who specializes in addiction research, testified in Nassau County Court that he had reason to believe, based on blood test results, that Beer was a chronic marijuana user.

Beer, 19, of South Richmond Hill in Queens, faces up to 25 years in prison if convicted of aggravated vehicular homicide. Prosecutors have said he was high on marijuana and speeding without a driver's license in October 2012 when he crashed his car in Lakeview.

The wreck killed Neal Rajapa, 17, and Peter Kanhai, Christopher Khan and Darian Ramnarine, who were 18. They were childhood friends of Beer's from Queens.

Sofuoglu also testified that his research was inconclusive about whether marijuana use increases the risk of car crashes.

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During cross-examination, Sofuoglu told Assistant District Attorney Michael Bushwack the fact that a person smoked $20 worth of marijuana before driving didn't necessarily mean he'd get in a crash. A prosecution witness previously testified that Beer said he smoked that much a few hours before the wreck.

Sofuoglu agreed with Bushwack that a person didn't have to show outward effects of a drug in order to be impaired. Sofuoglu said he has recommended in his writings that people wait three hours after smoking marijuana before driving.

Defense attorney Todd Greenberg rested his case after the testimony. Closing arguments are scheduled for Monday in Mineola.